Here is how to get a lot of lives! First get the feather on you and in that storage box. Then you go to yoshi's island and keep flying until you find a moon. It gives you 3 level ups and also there is another way!
Here is another way to get a lot of lives go to the donut plain 1 and go into the secret area where you can get the feather...get it and then you run up, hit the coin meter and keep flying this should give you about 6 lives.
These cheats have been tested and I got 140 lives by using these cheats.